Substance is an open source and web-based application for authoring and publishing documents.

It offers a MS Word-like-yet-simpler and easy to use interface for creating, editing and organizing content.

Any number of other users can be invited for collaborative editing where it will be possible to view which user created which parts of the document.

Users can subscribe to document for getting notified about the updates and comment on any parts of the content (and they can be seen in a nice toggled view).

The "table of contents" are created automatically and helps browsing long documents faster.

Also, an export feature can convert documents into many other formats including PDF, ODT, RDF, EPUB, HTML, Markdown and more.

Even if the Substance is not used as is, the modules (document editing, exporting, etc.) of the application are very handy to be used/integrated in other apps.

Similar to the popular disposible chat websites (check 7 Free Disposable Chat Systems For Instant Communication), Anologue is an impressive one which is open source.

It works by simply clicking on a "new room" link and the application instantly creates a chat room with a unique URL where you can invite others.

The application is totally Ajaxed, built with PHP and uses the Lithium Framework, CouchDB and jQuery.

And, if you don't prefer to install your own, you can still use Anologue as the hosted version is free for everyone.